The separation of various components found in liquids or gases may be effected in a multitude of processes, the techniques for effecting the separation utilizing asymmetric or composite membranes including selective permeation, ultrafiltration or reverse osmosis. A particular example of the latter type of separation involves a desalination process in which water is rendered potable or suitable for other purposes having been obtained from sea water, contaminated water, brackish water or brine. This process is of especial value in areas of the world where the water found in the area is brackish or is saline in nature. The desalination of this water is necessary in order to provide large amounts of potable or relatively nonsalty water for industrial, agricultural or home use. The desalination of the water is effected by forcing the water through a reverse osmosis membrane whereby the purified water is passed through the membrane and recovered, while the contaminants or salts do not pass through the membrane, thus, in effect, being rejected by the membrane and recovered as the retentate.
A reverse osmosis membrane, in order to be utilized for such a purpose, must possess certain characteristics applicable to the process. For example, the membrane must have a very high salt rejection coefficient. In addition, another important characteristic and a problem which must be addressed when utilizing the membrane, is the ability of the membrane to be tolerant to chlorine attack. Another important factor which is present in the use of a reverse osmosis membrane is that said membrane also possesses a high flux characteristic, that is, the ability to pass a relatively large amount of water through the membrane at relatively low pressures. If a membrane possesses these desirable characteristics, it will be commercially feasible in its applicability to the desalination process.
Reverse osmosis membranes have been prepared and used from a wide variety of known polymeric materials. While many of these polymeric materials possess the ability of reducing the concentration of a solute to where the rejection capability is in excess of 98%, some do not possess the necessary flux whereby the volume of water which is required to be produced by the membrane per unit of membrane surface per unit time is sufficient for the application of the technology.
Inasmuch as the semipermeable membrane which is used for the desalination process should be relatively thin in nature in order to provide a desirable flux, it is necessary, in many instances, that the reverse osmosis membrane be composited or laminated on a porous support backing material. This porous support backing material should in itself possess certain characteristics which make it desirable for such a use. For example, the porous support material should possess pore sizes which are sufficiently large enough so that the water or permeate can pass through the support without affecting or lessening the flux of the entire composite. Conversely speaking, the pore size should not be so large that the thin composite semipermeable membrane will tend to fill up or penetrate too far into the pores, thus distorting the shape of the thin film membrane with the attendant possibility of rupturing the membrane when operated under high pressure, thus causing said membrane to lose its effectiveness in the reverse osmosis process.
Many U.S. patents describe various membranes which are useful in desalination processes, see for example, those cited and discussed in U.S. Pat. No. 4,830,885. One of the earliest patents to describe membranes of the type used in the present invention is U.S. Pat. No. 3,744,642 to Scala et al. Scala et al. suggest reacting a broad group of amines or bisphenols with acyl halides or sulfonyl halides on a support material to form thin membranes. This provides strength to the composite.
U.S. Pat. No. 4,277,344 discloses a reverse osmosis membrane made in situ according to Scala et al., which has been prepared from a polyacyl halide and an arylene polyamine. The U.S. Pat. No. '344 teaches that the membrane contains a plurality of sites having the formula: EQU Ar(CON--).sub.2 COOH
in which Ar represents the aromatic nucleus residue of the polyfunctional aryl halide. It is of interest that according to the U.S. Pat. No. '344, solvents for the polyacyl halides that dissolve or plasticize the support material should not be used. In accord is U.S. Pat. No. 4,619,767 to Kamiyama et al. which states that it is necessary to avoid solvents for the crosslinking agents (e.g. acid halides) which dissolve or swell the porous substrate.
In U.S. Pat. No. 4,830,885 an improved supported membrane is disclosed in which a polyhydric compound (which does not dissolve typical support materials), is included with the amine solution in preparation of the membrane. The polyhydric compound provides improved flux through the membrane while maintaining the high rejection of the dissolved salts.
The high flux semipermeable membranes are sensitive in nature and require delicate and complex handling in order to avoid a rupture of the thin film, thus rendering the membrane inoperable. Usually the membrane must be maintained in a wet state in order to maintain the integrity of the film. For example, previously cited U.S. Pat. No. 3,744,642 states that the membrane is stored in a high humidity atmosphere or water so that the membrane does not dry out, while U.S. Pat. No. 4,830,885 teaches that the membrane is either kept wet or is treated with a polyhydric compound such as glycerine in order to protect the thin film membrane from drying out which would lead to a loss of performance of the membrane when used in a separation process. Commercial membranes usually can not be dried at room temperature or elevated temperatures (which may be defined as a weight loss less than about 2% after heating at a temperature of 110.degree. C. for a period of 1 hour) without imposing a deleterious effect upon the flux and rejection rate of the membrane. In order to facilitate a less complicated procedure for handling the membranes it is essential to have dry membranes which are less susceptible to damage due to the handling of the membrane, render the membrane easier for heat sealing, have less salt passage in elemental construction and possibly require no sterilization. As will hereinafter be shown in greater detail it has now been discovered that membranes which are formed according to the process hereinafter set forth in greater detail may be subjected to treatment with certain types of acids to form a membrane which may be dried and thereafter, when in use as a separation membrane, still maintain a high flux with a concurrent high rejection rate.